Controlled Construction of Nanostructured Organic–Inorganic Hybrid

Research Article pubs.acs.org/journal/ascecg

Controlled Construction of Nanostructured Organic−Inorganic Hybrid Material Induced by Nanocellulose Qilin Lu,† Zhenghan Cai,† Siqun Wang,‡ Fengcai Lin,† Beili Lu,† Yandan Chen,† and Biao Huang*,† †

College of Material Engineering, Fujian Agriculture and Forestry University, No. 15 Shangxiadian Road, Cangshan District, Fuzhou 350002, China ‡ Center for Renewable Carbon, University of Tennessee, Knoxville, Tennessee 37996, United States ABSTRACT: Sticky rice lime mortar (SRLM) is a widely used lime-based gelled material in ancient China. Sticky rice plays an important role in the formation of organic−inorganic hybrid material. Inspired by this, a nanostructured organic−inorganic hybrid material (NOHM) was constructed based on the regulation of nanocellulose (NCC) on the crystallization of calcium carbonate. The used NCC was prepared by a green way via simultaneous ball milling and phosphotungstic acid catalyzed hydrolysis. The characteristics of the prepared NOHM were explored to gain insight into the interaction of NCC and mineral during the mineralization of calcium carbonate. The results indicate that nanocellulose plays a crucial role in the formation of nanostructured organic−inorganic hybrid material, which not only induces and regulates the mineralization process of calcium carbonate crystals but also affects the morphology and size of the formed calcium carbonate crystals. The mechanical strength and durability of NOHM were significantly improved with the regulation of NCC. The results not only suggest new hypotheses about the role of polysaccharide in the mineralization process but also serve to shed light on the formation process of organic− inorganic hybrid material regulated by polysaccharide. KEYWORDS: Sticky rice lime mortar, Nanocellulose, Regulation, Mineralization, Greener process, Hybrid material

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INTRODUCTION Cellulose, the most abundant renewable biopolymer available on the earth, is widely used in commercial materials due to its attractive properties, such as biocompatibility, biodegradability, and thermal and chemical stability. Nanocellulose (NCC) is referred to as a bundle of nanosized fibrils, which has single digit nanometer in width dimension as well as hundreds to thousands of nanometers in length, with its dimensions being similar to that of carbon nanotubes.1 Compared with natural cellulose, NCC has many extraordinary advantages, such as high surface area and high strength. Thus, it has attracted a great deal of interest in composite manufacturing and material science due to its mechanical strength, low density, unique morphology, as well as biodegradability and environmental benefits.2 Sticky rice lime mortar (SRLM), a kind of organic−inorganic hybrid material, was a wonderful lime-based gelled material in ancient China and even in the history of world construction, which can be regarded as one of the great inventions of ancient China.3 It should be mentioned that the use of SRLM in ancient China can be dated back to 386 AD, which was widely used in palaces, tombs, city walls, and water conservancy projects because of its excellent properties, such as good adhesive strength, toughness, and waterproofness.4 The fact that these buildings remained intact and stable against thousands of years of wind and rain has proved the fine © XXXX American Chemical Society

strength and durability of the traditional lime-based binding material. Thanks to its high strength, good toughness, and superior antiseepage capability, SRLM can also be called “Chinese concrete”. In consideration of its outstanding advantages, specifically its durability and compatibility with the original structures, SRLM has been widely used in cultural relic conservation. Organic−inorganic hybrid materials are fascinating since they can make materials own excellent features. The development of such materials, which have already found numerous applications, is one of the great achievements of material science.5 Biomineralization is a natural process that finally leads to the formation of complex biomolecular−inorganic hybrid materials such as teeth, bone, and nacre, which all tend to have many superior mechanical properties to synthetic material hybrids.6,7 These special structures are assembled by highly controlled growth mechanisms at the organic/inorganic interface.8 Thus, bioinspired synthesis of crystals with complex forms that mimic natural biominerals in the presence of organic templates or additives has been intensively developed in the past years.9 In the many studies that set out to investigate the mechanisms of biomineralization processes, proteins, glycoproteins, polysacReceived: July 17, 2017 Revised: August 12, 2017 Published: August 18, 2017 A

DOI: 10.1021/acssuschemeng.7b02394 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering charides, and/or synthetic substrates were used to mediate mineral formation.10,11 It has been shown that the morphology, crystallographic polymorph, and structural properties of calcium carbonate can be controlled by the use of specific organic templates or additives.12,13 However, it is still unknown whether the nanocellulose as a natural polysaccharide may dominate the mineralization process of calcium carbonate. To address this question, organic−inorganic hybrid material was prepared, and the effects of NCC on the morphology and crystallographic polymorph formation process of calcium carbonate were examined. The study indicates that NCC has considerable implications for the mineralization process and structural formation of calcium carbonate. The constructed nanostructured organic−inorganic hybrid material has high mechanical strength and durability, and consequently potential application in advanced functional materials, such as, hydroxyapatite, biomimetic materials, bioceramics, etc.

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WCaCO3% =

MCaCO3 WCO2% MCO2

(1)

where MCaCO3 and MCO2 are the molecular weight of CaCO3 and CO2, respectively. Morphological Analysis. The morphology and size of the prepared nancellulose were analyzed by transmission electron microscopy (TEM) with a Hitachi-H7650 transmission electron microscope (Hitachi, Ltd., Japan), at an accelerated voltage of 100 kV. The dimensions and aspect ratio of NCC were determined by atomic force microscope (AFM) with a NanoScope IIIa MultiMode (Veeco Instruments, Inc., U.S.A.). The morphology and microstructure of hybrid material were observed by field emission scanning electron microscopy (FESEM) with a XL30 ESEM-FEG model FESEM (FEI Co., Ltd., USA). For every test, 30 mg of specimen was sputtered and coated with gold before observation. X-ray Diffraction (XRD) Analysis. The crystal habit and polymorph type of the hybrid material were investigated by XRD analysis. The XRD measurement was performed on an X’Pert Pro MPD X-ray diffractometer (Philips-FEI, Netherlands) with Cu Kα radiation at 40 kV and 30 mA. Diffractograms were collected in the range of 2θ = 6−90° at a scanning rate of 0.1° s−1. Mechanical Strength Analysis. Before test, all of the samples were conducted carbonation in natural indoor conditions at a relative humidity of 50%−75% and temperature of 20−25 °C for 30−150 days. As for the surface hardness test, the well-blended lime mortar was put in the cylindrical mold with a diameter of 50 mm and height of 15 mm (28.4 g per piece of block) and then vibrated to make the mortar blocks become compact and flat. The surface hardness of the prepared hybrid material was measured with a Shore D Durometer. The compressive strength testing cubes were prepared with 50 mm × 50 mm × 50 mm (123.6 g per piece of block) cubic mold. The testing cubes were fastened to the sample holder, making the upper surface thoroughly contacted with the pressure probe. The pressure regularly increased at a rate of 0.01 MPa s−1, and the compressive strength value was identified as the largest value when the cubes were destroyed. The durability of the prepared hybrid material was characterized by freeze− thaw cycle analysis. The well-blended lime mortar was put in the cylindrical mold with a diameter of 50 mm and height of 15 mm (28.4 g per piece of block) and then vibrated to make the mortar blocks become compact and flat. All of the samples were immersed into deionized water for 12 h at 25 °C, and then by the soaked samples were frozen at −20 °C for 12 h and then taken out and immediately put into deionized water to thaw. After 12 h thawing in water, the samples’ surfaces were observed and recorded, followed by immersion into deionized water again. This cycle of procedures was repeated for 13 times. The index of cycles when the samples’ surfaces were cracked was identified as the index of resistance to freeze−thaw cycles.

EXPERIMENTAL SECTION

Materials. Bamboo pulp was supplied by Nanping Paper Co., Ltd. (Nanping, Fujian, China), and phosphotungstic acid (PTA), calcium hydroxide and sticky rice were purchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). All chemicals used in this work were of analytical grade without any further purification. Extraction of Nanocellulose. Nanocellulose was obtained via phosphotungstic acid hydrolysis, according to our previous literature.14 The bamboo pulp was cut into pieces and beaten to form cellulose pulp with a Fiber Standard Dissociation device for 20 min at 1500 rpm. Mechanochemical activation of cellulose pulp was performed within a ball mill equipped with two 90 mL agate jars, each of which was loaded with 20 6 mm-diameter agate balls. For each experiment, a mixture of 1 g of cellulose pulp and 20 g of 13 wt % phosphotungstic acid solution was added into the agate jar, and then the mixture was subjected to milling at 400 rpm for 2 h. After ball milling, the balls were removed from the resulting sample, which was then introduced into a round-bottom flask equipped with a condenser and kept at 90 °C in an oil bath for 4 h. The resulting mixture was purified with deionized water by repetitive centrifugations at 9000 rpm for 10 min. The precipitate was processed by ultrasonication treatment at 20 kHz for 0.5 h in an ultrasonic reactor prior to the obtention of nanocellulose. Construction of Nanostructured Hybrid Material. A total of 5 g of sticky rice was added into the as-prepared nanocellulose solution and mixed well to form 500 mL of 6 wt % mixture of sticky rice and nanocellulose (5 wt % sticky rice and 1 wt % nanocellulose), and then the mixture was boiled continuously for 2 h in an oil bath; the volume of the mixture was maintained by adding deionized water during the process of boiling. Calcium hydroxide was then added into the mixture which was stirred at 500 rpm for 2 h, followed by casting and carbonation at the relative humidity of 50%−75% and a temperature of 20−25 °C for 30−150 days to form the NOHM. It should be pointed out that the weight ratio of sticky rice/Ca(OH)2 and H2O/Ca(OH)2 were controlled at about 0.05 and 1.27, respectively. The organic− inorganic hybrid material was shaped into a cylinder and cube. Component Analysis. The polymorph determination of hybrid material was analyzed by Fourier-transformed infrared spectra (FT-IR) and thermal gravimetric analysis (TGA). FT-IR spectra of the samples were studied with a Nicolet 380 FT-IR spectrometer (Thermo Electron Instruments Co., Ltd., U.S.A.) in the frequency range of 4000−400 cm−1 with a resolution of 4 cm−1. Prior to analysis, 1 mg of the sample was first ground with 100 mg of KBr and pressed into thin pellets. The thermal characteristics of hybrid material were analyzed with a thermal gravimetric analyzer (NETZSCH STA 449 F3 Jupiter). A total of 10 mg of specimen was put into an alumina crucible and heated from 30 to 1000.°C at a heating rate of 10 °C min−1 in nitrogen atmosphere at a flow rate of 30 mL min−1. The content of calcium carbonate (WCaCO3%) in the hybrid material can be calculated by the percentage of carbon dioxide release, according to the following eq 1:

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RESULTS AND DISCUSSION Component Analysis. The FT-IR spectra of SRLM and NOHM are presented in Figure 1. The band at 3430 cm−1 represents the O−H stretching vibration of water absorbed by the surface of calcium carbonate, and 1790 cm−1 represents the CO stretching vibration of calcium carbonate. The absorption bands at 1790, 1440, 874, and 712 cm−1 are the characteristic absorption peaks of calcite, suggesting that the main crystal form of the prepared NOHM is calcite. The bands at 874 and 712 cm−1 are ascribed to the deformation vibration of C−O of the carbonate groups of calcite, and the band at 1440 cm−1 is ascribed to the asymmetrical stretching vibration of C−O of calcite.15 All of these bands can also be found in the FT-IR spectrum of SRLM, indicating that the crystal form of calcium carbonate in SRLM is similar to that of the prepared NOHM which belongs to calcite. However, as for NOHM, the new absorption bands at 2913 and 2852 cm−1 are detected, which represent the stretching vibration of methylene, suggesting that the organic component of NCC combined B

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inorganic component of NOHM is mainly calcium carbonate, but because of the low diffusion rate of carbon dioxide in the air, the carbonation degree of calcium carbonate is not complete at a certain curing time, and a small part of calcium hydroxide (about 10%) still exists. Morphological Analysis. As can be seen from the TEM (Figure 3) and AFM images of NCC (Figure 4), NCC shows short rod-like structures, and these nanoparticles form a network structure which can act as the strengthening phase in composite materials.18 The particle size of NCC is calculated and find that these nanocrystals are 200−400 nm in length and 30−50 nm in width. Moreover, the lower degree of agglomeration can be attributed to the strong hydrogen bonds among NCC, which results in the formation of selfassembled networks.19 Similar results can be found in other literature, which reported that the nanocellulose prepared by sulfuric acid hydrolysis formed self-assembled networks on account of the strong H-bonding among nanocrystals. However, the nanocellulose isolated from bamboo pulp was also reported in the forms of spherical or fibrils. The differences in the morphologies of nanocellulose could be caused by different preparation methods. Figure 5 shows the SEM images of SRLM (Figure 5a) and NOHM (Figure 5b). As for SRLM, the shape of the formed calcium carbonate crystals is rhombohedral with the size of 300−400 nm, and the combination among crystals is relatively loose. Compared with SRLM, the shape of the formed calcium carbonate crystals under the control of NCC is rod-like with a width of 50 nm and length of 100−150 nm. In addition, these calcium carbonate nanocrystals combine closely, leading to the formation of a dense structure. The more compact the structure is, the higher the mechanical strength of NOHM is. The changes in morphology and particle size for the calcium carbonate crystals of NOHM are attributed to the specific regulating function of NCC on the mineralization process of calcium carbonate.20 Under natural conditions, calcium carbonate crystals grow to be rhombohedral calcite. With the addition of NCC, Ca2+ ions combine with the hydroxyl groups of NCC through electrostatic attraction and chelation, leading to the changes of growth direction and growth rate of crystals, and results in the change of morphology of calcite.21,22 During the growth of calcium carbonate crystals, the spatial structure of NCC may be a key factor to the morphology of calcite formation. NCC interacts with calcium carbonate selectively in a specific direction and forms space network structure, followed by controlling the carbonation process. In the inside of the space network structure, NCC both provides structural framework upon which calcium carbonate crystals grow and serves as a source of chemical functionalities to direct nucleation and growth of the crystals.23 Furthermore, the electrostatic interaction and steric hindrance of NCC hinder the sustained growth of calcite crystals, leading to the formation of calcite nanocrystals and the decrease of particle size. X-ray Diffraction (XRD) Analysis. The XRD patterns of NCC are shown in Figure 6a. NCC presents four diffraction peaks at 2θ = 15°, 16.5°, 22.7°, and 34.8°, corresponding to the

Figure 1. FT-IR spectra of NOHM.

with calcium carbonate by chemical bond combination in the prepared hybrid material. These results indicate that during the formation of organic−inorganic hybrid material, chemical actions take place between NCC and calcium carbonate, that is, NCC interacts with lime by ionic bonds to form space net structure. In the inside of the space net structure, CO32− interacts with Ca2+ by certain direction to form calcium carbonate, and the shape and size of the formed space net structure regulate the morphology and particle size of the obtained calcium carbonate during the process. Because the carbon dioxide diffusion rate is low in the air, the carbonation rate of lime is slow and the formation of hybrid material needs much time. Table 1 and Figure 2 show the thermogravimetric analysis results of NOHM. As can be seen from the thermogravimetric results, the thermal decomposition of SRLM and NOHM can be divided into three steps. The first step is attributed to the decomposition of organic materials of sticky rice and/or NCC in the range of 200−400 °C, while the second step in the range of 400−600 °C is due to the thermal decomposition of calcium hydroxide, and the third step is associated with the decomposition of calcium carbonate in the range of 600−800 °C.16 In the first step of thermal decomposition, the weight losses are 5% and 6% for SRLM and NOHM, respectively, due to the thermal decomposition of glycosyl units of sticky rice and/or NCC.17 In the third step of thermal decomposition, the temperature of maximum weight loss rate is 700 °C for SRLM, and by contrast, the temperature of the maximum weight loss rate is 660 °C for NOHM, which means NCC has an interaction with calcium carbonate and is one of the reasons for the thermal decomposition of calcium carbonate. As for SRLM and NOHM, the weight losses between 600 and 800 °C are 26.4% and 33.66%, respectively, corresponding to the contents of calcium carbonate of 60% and 76.5%, respectively, indicating the higher carbonation degree of NOHM with the regulation of NCC. The analytical results described above show that the

Table 1. Thermogravimetric Analysis Results of SRLM and NOHM samples

decomposition temperature (°C)

weight loss (%)

decomposition temperature (°C)

Tmax (°C)

weight loss (%)

CaCO3 content (%)

SRLM NOHM

200−400 200−400

5 6

600−800 600−800

700 660

26.4 33.66

60 76.5

C

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Figure 2. (a) TGA curves of SRLM and NOHM and (b) DTG curves of SRLM and NOHM.

Figure 3. TEM micrograph of NCC.

(1−10), (110), (200), and (004) crystallographic planes of the monoclinic cellulose Iβ lattice, indicating that the crystalline type of NCC is similar to that of native fiber.24 Compared with native fiber, the crystallinity index (CrI) of NCC increases from 63.7% to 79.6%, which is due to the degradation of amorphous regions and disordered regions of cellulose in the hydrolysis process.25 Higher crystallinity index in NCC is associated with higher tensile strength and thermal stability, which is expected

Figure 5. (a) SEM image of SRLM and (b) SEM image of NOHM.

to be beneficial for producing high-strength composite materials.26 The XRD patterns of SRLM and NHOM are shown in Figure 6b. It is obvious that the crystalline phase mostly consists of calcite phase for both SRLM and NOHM.

Figure 4. AFM images of NCC. D

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Figure 6. (a) XRD patterns of NCC and (b) XRD patterns of NOHM.

Figure 7. Schematic representation of the calcite crystals formed in NCC solution.

vertical direction of the crystal plane and making the crystal plane remain stable.27,28 However, the interaction between organic matter and inorganic matter cannot hinder the growth of other crystal planes, so the crystal plane of (104) is still calcite crystal in the formed calcium carbonate. In addition, as for both SRLM and NOHM, the diffraction peaks of calcium hydroxide can also be detected, implying the existence of calcium hydroxide and indicating the incomplete carbonation for SRLM and NOHM at a certain curing time, which is in agreement with the above-mentioned thermogravimetric analysis results. It is generally accepted that polymers with polar functional groups are favorable to heterogeneous

Additionally, the peak intensity of calcite for NHOM is greater than that of SRLM, which demonstrates an increase of calcite content, indicating a higher carbonation degree. These results completely correspond with the FT-IR characterization. It is worth noting that the polymorph of the formed crystals is calcite which is the same as that of SRLM. In other words, the addition of NCC has not changed the crystallographic polymorph during the formation of calcium carbonate. As far as we know, when organic matter selectively interacts with a certain direction of crystal plane by molecular recognition, the growth position of crystals in the direction will be blocked, leading to a decrease or stop of growth rate of crystals in E

DOI: 10.1021/acssuschemeng.7b02394 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering nucleation of calcium carbonate crystals.29,30 As a kind of natural multihydroxyl polysaccharide, NCC can facilitate the mineralization process of calcium carbonate and regulate the morphology, structural properties, and particle size of calcium carbonate crystals.31 The calcium ions (Ca2+) are anchored along NCC chains both through electrostatic attraction and through chelation between hydroxyl groups and Ca2+ ions, leading to the increase of Ca2+ ions concentration on the surface of NCC chains. Subsequently, the anions (CO3)2− are attracted to the region where the concentration of Ca2+ ions is high, leading to the heterogeneous nucleation of CaCO3 nanocrystals and prompting the growth of calcium carbonate crystals. As is well-known, during the crystallization process, the crystallographic growth of the crystal following nucleation is much easier because, once the nucleus is formed, the activation energy required for further growth of the crystal becomes lower.32 Therefore, the polymorph and orientation of the formed CaCO3 crystals might be controlled by the conformation of the NCC molecules, which would be capable of binding the Ca2+ with its hydroxyl groups. Possible Regulation Mechanism of Nanocellulose on Polymorph and Morphology of CaCO3 Crystals. Based on the above results and analyses, a possible crystallization process for the rod-like calcium carbonate particles has been proposed (Figure 7). During the mineralization process, Ca2+ cations can combine with the negatively charged hydroxyl groups of nanocellulose by electrostatic interaction.33 In addition, the rod-like nanocellulose also provides a nucleus position for the crystallization of calcium carbonate. Initially, amorphous calcium carbonate is formed in the nanocellulose solution via heterogeneous nucleation. Then, the electrostatic interaction between Ca2+ ions and hydroxyl groups of the nanocellulose bound by the amorphous calcium carbonate controls the growth of these calcium carbonate crystals.34 Nevertheless, amorphous calcium carbonate is a metastable phase that cannot exist steadily and rapidly transforms into calcite in the nanocellulose solution.35 Nanocellulose plays a pivotal role in every period of mineralization including nucleation, growth, and aggregation. After initial crystallization, the nanocellulose solution controls the growth of calcite crystals. During the growth of calcite crystals, the hydroxyl group chains may attach to some specific crystal faces, and this adsorption leads to their enlargement. The morphology and orientation of the formed calcium carbonate crystals may be controlled by the match between the conformation of the hydroxyl groups and the crystal lattice of calcite.36 Mechanical Strength Analysis. The surface hardness test results of SRLM and NOHM are shown in Figure 8a. Compared with SRLM, with the addition of NCC, the surface hardness value of NOHM increases by 42% at a curing time of 150 days. Moreover, with the increase of curing time, the surface hardness value of NOHM increases from 51.2 HA for 30 days to 77.4 HA for 150 days. During the mineralization of calcium carbonate, NCC provides a structural framework upon which inorganic minerals grow and guides the orientation of the aggregation and the crystallographic growth of the calcite crystals. Meanwhile, these nanocellulose particles can associate with calcite crystals by the bonding of hydroxyl groups with Ca2+ ions, thereby forming composite materials of unique mechanical property. Figure 8b shows the compressive strength test results of SRLM and NOHM. With the addition of NCC, the compressive strength of NOHM increases by 93% at a curing

Figure 8. (a) Test results of surface hardness of NOHM, (b) test results of compressive strength of NOHM, and (c) test results of freeze−thaw cycle of NOHM.

time of 150 days. NCC itself has high strength, and thousands of nanocellulose particles are distributed inside the lime mortar, forming a complicated three-dimensional disorder system, which can enhance the strength of the formed NOHM.37 The formation of the network structure of nanocellulose F

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provides the fundamental condition to regulate the structure of the organic−inorganic hybrid material. The disorderly distribution of nanocellulose can serve as the skeleton and bond, making calcite crystals combine closely and forming a more dense structure, which can improve the mechanical strength of NOHM.38 Additionally, the formation of covalent bonds between calcium cations and hydroxyl groups of NCC can also help improve the bonding strength of calcite crystals.39 Figure 8c shows the freeze−thaw cycle test results of SRLM and NOHM. The test results show that the samples with NCC (NOHM) have better resistance to freeze−thaw cycles because they keep intact after 13 freeze−thaw cycles while the samples without NCC (SRLM) are destroyed after 6 freeze−thaw cycles, which means that the durability of NOHM is apparently better than that of SRLM because the property of frozen and thaw of NOHM increases by 117%. The main reason is that the empty cavity in nanocellulose can store free water and thus avoid the rapid evaporation of water in mineralization process.40 The water retention function of NCC can provide necessary water supplement at the early stage of mineralization process and promote the improvement of carbonation degree, leading to a more dense structure.41 In addition, the empty cavities created by nanocellulose actually provide internal reservoirs that accommodate ice crystal growth during the freezing segment and thus minimize the expansive pressure generated by the freeze of available water in capillary pores of NOHM, reducing the destruction to NOHM by expansive pressure and improving the resistance to freeze−thaw cycles.

REFERENCES

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CONCLUSIONS A novel nanometer-sized hybrid of nanocellulose and calcium carbonate, which has high mechanical strength, durability, and unusual rod-like shapes, is constructed. Nanocellulose as a natural polysaccharide plays a crucial role in the formation of nanostructured organic−inorganic hybrid material. NCC not only induces but also regulates the mineralization process of calcium carbonate crystals. The investigations on the characteristics of NOHM show that NCC can facilitate both the growth of calcium carbonate crystals and aggregation of crystals into superstructures, controlling the morphology and size of the formed NOHM. The fundamental cause of the excellent properties of NOHM lies in the regulation of NCC, favorable nanocrystal−matrix interactions, and the ability of the polysaccharide network. Additionally, the results displayed here possibly offer some new insights into the research on the mechanism of mineralization.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 591 88160598. Fax: +86 591 85715175. E-mail: [email protected]. ORCID

Biao Huang: 0000-0001-7363-0060 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the China Postdoctoral Science Foundation (Grant No. 2017M610387), Special Scientific Research Fund for Public Service Sectors of Forestry (Grant No. 201504603), and National Natural Science Foundation of China (Grant No. 31370560). G

DOI: 10.1021/acssuschemeng.7b02394 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acssuschemeng.7b02394 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX